Hitherto, various printing plates for performing lithographic printing by using a silicone rubber or fluorine resin as an ink repellent layer without using dampening water (hereinafter referred to as “waterless lithographic printing”) have been proposed. Waterless lithographic printing is a lithographic printing method in which an image area and a non-image area are made to be present on almost the same plane, and with an ink acceptable layer as the image area and with an ink repellent layer as the non-image area, the difference in ink acceptability is used for allowing the image area only to accept ink. Subsequently the ink is transferred onto a printing medium such as paper, to achieve printing. The lithographic printing method has a feature that printing can be performed without using dampening water.
As methods for exposing the waterless lithographic printing plate precursor, various methods are proposed. These methods can be roughly classified into methods of performing ultraviolet irradiation via an original film, and computer-to-plate (hereinafter referred to as CTP) methods in which the image is directly written from the text without using the original film. The CTP methods include a method of irradiating with a laser beam, a method of writing using a thermal head, a method of partially applying a voltage using pin electrodes, a method of forming an ink repellent layer or ink acceptable layer using ink-jet, and so on. Among these methods, the method of irradiating with a laser beam is more excellent than the other methods in view of resolution and plate making speed.
The method of irradiating with a laser beam can be classified into two types: a photon mode method using a photo reaction and a heat mode method in which photo thermal conversion is performed to cause a thermal reaction. In particular, the usefulness of the heat mode method grows higher owing to the advantage of allowing handling under daylight and the rapid progress of the semiconductor lasers used as light sources.
With regard to the directly imageable waterless lithographic printing plate precursors corresponding to the heat mode method, various proposals have been made hitherto. Above all, a directly imageable waterless lithographic printing plate precursor having bubbles in a heat sensitive layer is proposed as a directly imageable waterless lithographic printing plate precursor that allows plate making with low laser irradiation energy and is good in image reproducibility (for example, see patent document 1). Further, as a method for producing a directly imageable lithographic printing plate precursor that allows plate making with low laser irradiation energy and is good in image reproducibility, proposed is a method for producing a directly imageable waterless lithographic printing plate precursor, comprising a step of coating a heat sensitive layer composition solution containing an organic solvent with a solubility parameter of 17.0 (MPa)1/2 or less and a step of drying the heat sensitive layer composition (for example, see patent document 2).